[LibOS] Makefile: Remove OMIT_FRAME_POINTER flag

There is no code depending on it. If it's wanted, one could simply
change CFLAGS.

LibOS/shim/src/Makefile

@@ -2,8 +2,6 @@ RUNTIME_DIR = $(CURDIR)/../../../Runtime
 
 include ../../../Scripts/Makefile.configs
 
-OMIT_FRAME_POINTER = no
-
 CFLAGS += -fPIC -Winline -Wwrite-strings \
 	  -fmerge-all-constants -Wstrict-prototypes -Wtrampolines \
 	  -Werror=implicit-function-declaration \
@@ -14,11 +12,6 @@ CFLAGS += -fPIC -Winline -Wwrite-strings \
 
 CFLAGS += -Wextra
 
-ifeq ($(OMIT_FRAME_POINTER),yes)
-CFLAGS += -DOMIT_FRAME_POINTER=1
-else
-CFLAGS += -fno-omit-frame-pointer -DOMIT_FRAME_POINTER=0
-endif
 ASFLAGS += -Wa,--noexecstack -x assembler-with-cpp -I../include
 
 LDFLAGS += -shared -nostdlib -z combreloc -z relro -z now -z defs \